Conventionally, a so-called Kepler type observation optical system has been generally in practical use, which performs primary imaging of an object image to be an erect image via an objective optical system and an inverting optical system, and makes the image observable by an eyepiece system. For example, a finder optical system disclosed in Japanese Patent Laid-Open No. 10-10440 (Patent Literature 1) is mainly characterized in that it appropriately sets the distance between a primary imaging surface of an observed image and a lens provided at its pupil side. In addition, a finder optical system disclosed in Japanese Patent Laid-Open No. 2001-174716 (Patent Literature 2) is mainly characterized in that it secures the distance from an image formation position of an observed image to an optical component at the object side facing thereto, and the distance to an optical component at the pupil side to be equal to or larger than a predetermined distance. Both optical systems are intended to prevent an observer from recognizing dirt or scratches, if any, sticking to or formed on a lens or an optical component near the imaging position, by avoiding focusing on the dirt or the like on the optical surface.
However, only putting dirt or the like sticking to the optical surface out of focus by providing the optical surface outside a predetermined diopter range as described in Patent Literature 1 or 2 may be insufficient to prevent the dirt or the like from being recognized. For example, a larger ratio of the size of dirt sticking to the optical surface relative to the beam diameter may result in a shadowed observed image due to eclipse of the light beam or a lower contrast. Such a state may also be recognized by the observer, which does not provide an appropriate product quality.